Microstructural devices having a movable membrane for creating optical interference effects may be used in a variety of applications. For example, such devices are useful as high speed, inexpensive optical modulators for optical communications systems.
The theory underlying the performance and mechanical response of fiber optical microstructures configured for measuring pressure and temperature is generally described in Greywall et. al, U.S. Pat. No. 5,831,262 (hereinafter Greywall et. al '262). Additional description concerning the design of micromechanical optical modulators may be found in U.S. Pat. No. 5,500,761 and U.S. Pat. Nos. 5,654,819 and 5,589,974.
The microstructure may be suitably structured for example as a Fabrey-Perot device having equal reflectivity mirrors as generally described in Aratani' et al., "Process and Design Considerations for Surface Micromachined Beams for a Tunable Interferometer Array in Silicon", Proc. IEEE Micro. Electromech. Workshop, Ft. Lauderdale, Fla., Feb. 7-10, 1993, pp. 230-235.
Using such devices in optical systems requires optical coupling to waveguides such as optical fibers. Optical coupling, however, may be problematic. Greywall et al. described an Article Comprising An Optical Fiber Attached To A Micromechanical Device in Greywall et. al '262 in which an optical fiber is integrally attached to a microstructure. In Greywall et al. '262 a layer of cement and a layer of glass are index matched to the index optic fiber, i.e. the layers and the fiber have the same index of refraction. The glass layer provides support for an adjacent layer of the microstructure. Integration of the fiber with the microstructure was taught to eliminate or reduce the interference effects that would otherwise occur if the fiber end and the microstructure were spaced.
The fabrication of the microstructure as described in Greywall et al. '262 includes silicon nitride or polysilicon which is deposited on the first side of a silicon wafer. A "pill" of readily etchable sacrificial material is then deposited on the first layer. A second layer side composed of silicon nitride or polysilicon, is deposited on top of the pill and then a layer of glass is deposited over the second layer. The wafer is etched from the second side to the first layer. Also, holes are etched into the first layer through a conductive layer if present. The etchant is delivered through the holes to the pill of the sacrificial material sandwiched between the first and the second layer. The sacrificial material is etched away, releasing the first layer.
Since the manufacturing procedure of Greywall et al. '262 requires use of a pill of readily sacrificial material and removal of the sacrificial layer through the holes etched through the first layer, the "pill" must be precisely deposited over the first layer if the gap formed after the sacrificial layer is removed is to have the needed dimensions. In addition, the procedure includes etching holes in order to remove the sacrificial layer. The holes in turn may interfere with the structural properties of the movable membrane or may have to be plugged.
Greywall et al. '262 also describes that the terminal end of the fiber be connected to the flat surface of the glass via a layer of cement between the terminal end of the fiber and the glass. As such, the cement composition must be index matched and have suitable bonding properties to attach the fiber to the glass.